Charge-density analyses of the antiferromagnetsNiF2andFeF2by means of γ-ray diffraction

Accurate single-crystal diffraction data have been collected from ${\mathrm{NiF}}_2$ at room temperature and in the antiferromagnetic state at 15 K, and from antiferromagnetic ${\mathrm{FeF}}_2$ at 11 K. The γ-ray wavelength used was λ=0.0392 Å, and the maximum value of sin θ/λ was 1.40 A${\mathrm{̊}}^{\mathrm{-}1}$. Different wavelengths have been applied to correct the strongest reflections of ${\mathrm{NiF}}_2$ for extinction. On magnetic ordering in ${\mathrm{NiF}}_2$ a shift of the fluorine ions of 3×${10}^{\mathrm{-}3}$ Å is found; this confirms a prediction deduced from optical birefringence. In all cases, the centroids of the atomic charge density agree excellently with the nuclear positions determined by neutron diffraction. The static charge density is described by a rigid pseudoatom model; the resulting occupancies of the metal 3d orbitals agree with expectations from crystal-field theory; magnetic ordering hardly affects the density deformations in ${\mathrm{NiF}}_2$. The critical points of the chemical bonds have been determined to obtain information about possible covalency.